1. Field of the Invention
The present invention relates to a copper installation pipe having a tin coating that adheres to the entire inner surface, as well as to a method for coating a copper pipe.
2. Description of the Prior Art
Copper installation pipes have proven to be successful for all supply lines in house installations, in particular, for those used in supplying of drinking water. To reduce the amount of copper ions released into the water, seamlessly drawn copper pipes having a tin-coated inner surface are sometimes used. Installation pipes of this type meet the certain legal or regulatory requirements for all drinking water, with respect to the maximum limit of copper ions released into the water of 3mg/l following twelve hours of stagnation in the pipe conduit.
A conventional method for coating the interior surfaces of copper pipes with tin is the chemical deposition of tin on the interior surface of the copper pipes, as described, for example, in U.S. Pat. No. 2,282,511.
To this end, the copper pipe is rinsed with a chemical tinning solution. The deposition of tin takes place then by simple chemical metal displacement (or ion exchange). Copper ions are released from the base metal and, at the same time, an equivalent amount of tin ions are precipitated out of the tinning solution. This takes place under the influence of a potential existing between the tinning solution and the copper. There is no need for an external voltage or an electric current to create this potential.
The advantages of chemical tinning are manifested in its simple method of operation, with comparatively low expenditure for equipment, as well as in the possibility of depositing tin on the insides of the copper pipes, which otherwise are difficult to access.
What is disadvantageous, however, with chemical tinning is the fact that the tin ions are exchanged for copper ions as a function of the potential difference existing between the tinning solution and the copper pipe. Therefore, the deposition rate decreases as a function of the applied coating thickness, resulting in production of an inhomogeneous layered structure. The reaction comes to a standstill, as soon as the base metal is completely covered by the coating.
It can also be ascertained that the tin crystals grow with relative positional disorder during the tinning process. This produces a large, exposed tin layer surface, promoting the release of tin ions into the water during later use of the installation pipes. This also has an adverse effect on the corrosion resistance of the tin layer.
Due to fluctuations in the quality of drinking water, as often occur in practical, everyday use, one endeavors to further reduce the release of copper ions into the water intended for human consumption, by tin-coating the interior of the copper pipes. At the same time, the release of tin ions in the internally tin-coated copper pipe must be minimized. This applies, in particular, to water having a low pH-value or a large concentration of free carbonic acid (K.sub.B 8.2 &gt;1.0 mol/m.sup.3).